The present invention relates generally to means for testing electrical circuit boards, and more specifically to probe apparatus for facilitating operational analysis of electrical circuit boards in place and under actual operating conditions.
Various techniques and forms of apparatus have been devised for checking out and analyzing the electrical performance of circuit boards now commonly employed in the construction of electrical and electronic equipment. The continuing trend in such testing is toward processing samples of signals under investigation directly at the point of signal interception. This trend is demonstrated by a variety of recently introduced test apparatus, of which an example is the Hewlett-Packard Model 548A Logic Clip.
It is obviously necessary that any circuit board testing technique or apparatus be compatible with the overall design and construction of the equipment of which the circuit board is a part. Electronic equipment of any complexity commonly includes a number of circuit boards which are integrated with remaining portions of the equipment by means of edge connectors. Groups of circuit boards may be compactly housed in a drawer, side by side in parallel planes. Stationary portions of a plurality of edge connectors are typically located at the back of the drawer on or adjacent a back plane or ground plane. The circuit boards are plugged into the stationary portions of the edge connectors and may be withdrawn from the front of the drawer.
In general, circuit board testing and check out techniques compatible with the above-described construction can be grouped into the following categories:
(1) Use of special test apparatus for testing circuit boards apart from the electronic equipment of which they are components.
(2) Use of a card extender or patch cable between a circuit board and mating portions of the host equipment to bring nodes and components on the board into a position where they can be reached with an external test probe, etc.
(3) Use of built in test points and/or indicators located at a conveniently accessible edge of the circuit board or other convenient location.
Each of the noted techniques has strong points and inherent disadvantages. For example, apparatus for testing circuit boards apart from the host equipment generally has the capability of accessing most or all nodes and/or electrical components on a board. However, the apparatus required for this method of testing is generally complicated, expensive and special purpose in nature. Typical examples are shown in U.S. Pat. Nos. 3,564,408 and 3,970,934 issued respectively to F. G. Schulz et al on Feb. 16, 1971 and A. Aksu on July 20, 1976, and in published articles such as Schwedner et al, "In-Circuit Testing Pins Down Defects in PC Boards Early", Electronics, pages 98--102 (Sept. 4, 1975). Additional inherent disadvantages are that the circuit board under investigation is removed from its normal location and environment, and is electrically powered from the testing apparatus which may supply power which differs in regulation, noise, etc. from that supplied by the normal working power supply. Further, the circuit board is supplied with test signals which are not necessarily identical to signals encountered in normal operation. The result is that the particular combination of environmental and signal characteristics which result in a malfunction of the circuit board may not be duplicated during the test procedure.
The card extender technique requires only relatively simple and inexpensive special purpose devices in combination with general purpose test instrumentation or a probe such as previously identified Logic Clip 548A. This technique has the additional advantage that the circuit card is powered and supplied with the same signals involved in normal operation. The circuit board is, however, removed from its normal working location and environment, and may be subjected to abnormal conditions of temperature and electrical and magnetic fields, etc. In addition, probes and/or conductors for sampling signals at the circuit nodes and carrying test signals may introduce spurious signals and/or provide distributed capacitance which disturbs normal circuit board performance.
The use of built-in test points and/or indicators has the advantage that electrical performance of a circuit board can be analyzed with the board in its normal working location and environment. In addition, special external test apparatus is normally not required. However, this technique requires additional components, conductors, indicators, etc. on each circuit board, thereby effectively limiting the number of test points. In addition, the test points are permanently assigned. Such test points may not be those of principal interest when searching for the cause of a particular malfunction. Accordingly, this technique lacks flexibility.
The applicant has devised a unique method and apparatus which permits circuit board operation to be analyzed with the board in its normal location and environment, and permits access to any node on the board. Operational analysis of a circuit board with the board operating in its normal place and under normal conditions is known as in situ testing. The test apparatus which is relatively simple and inexpensive, provides electrical decoupling in close proximity to any node supplying signals for analysis to prevent the introduction of signal disturbances. Accordingly, many of the disadvantages of prior art circuit board testing techniques and apparatus are reduced or eliminated.